We propose to combine the best features of Positron Emission Tomography (PET) and Magnetic Resonance Imaging (MRI) modalities in a single instrument that will simultaneously record data in both imaging modalities. Moreover, we will develop labeled probes that can be detected in both PET and MRI to aid in the interpretation of complex biological processes. This system will be dedicated to the study of small animal model systems at the highest spatial and temporal resolutions attainable. We will build a high resolution, relatively high sensitivity multi-slice muPET scanner integrated within a customized 7T/30cm small animal MR system that will simultaneously record MR and PET images Through the use of fiber-optic couplings, the muPET system will interfere minimally with the muMR system, enabling high quality muPET and IJMRI data to be acquired essentially simultaneously and in near-perfect spatial registration. This system is a natural extension of earlier proof-of-principle systems and a newer prototype animal muPET system now nearing completion. The basic design elements of the system have been tested and demonstrated to work. The combined system we propose adds a number of important features to improve performance and ease of use for in vivo imaging studies. It also incorporates, for the first time, a multi-slice muPET system, with four detector rings simultaneously providing seven imaging planes, spanning an axial field of view of approximately 8mm, with at least 2mm resolution. Simultaneous muPET/muMRI recordings will provide important correlations not available from temporally and spatially separate scans (e.g. BOLD MRI compared with FDG PET). The melded system will provide high resolution anatomical reference systems for muPET studies The 'in register' muMR images will be used to compute scatter and attenuation in the IJPET images and to estimate partial volume errors in the PET scans, thus aiding quantification of the PET signal. This system will open up a number of opportunities not possible with current independent technologies Among them are: Time correlated IJPET and MRS studies of drug distributions; cardiac, CNS and tumor cell metabolism. Simultaneous fMRI and muPET neuroreceptor brain mapping studies in small animals. Validation of new MRI probes using their PET counterparts. Dual PET/MRI labels will allow for "zooming-in" the MRI data collection scheme to those regions of the specimen containing the label, as well as providing for precise registration of the PET & MR images.